Osmosis is a natural phenomenon that occurs when water from a less concentrated solution diffuses through a semipermeable membrane to a more concentrated solution. The semipermeable membrane is selective, i.e., allowing water molecules to pass while retaining total dissolved solids ("TDS").
Reverse osmosis is a phenomenon that is achieved when pressure is supplied to a concentrated solution in order to drive water through a semi-permeable membrane and away from the concentrated solution, against osmotic pressure. The dissolved and particulate matter in the concentrated solution remain behind. The proportion of salts to water in the concentrated solution will therefore increase, with a corresponding increase in the osmotic pressure. Thus, the higher the percentage of pure water that is extracted from the salt solution, the greater the pressure that must be exerted on that solution.
Reverse osmosis ("RO") has evolved into a common, reliable, and economical method to purify water. This treatment method purifies water by continuously concentrating and removing contaminants from a feed stream with relatively low energy and chemical usage.
In operation, it is important that the RO system be sampled periodically in order to generate a conductivity profile along the length of the pressure vessel. This sampling is performed in order to assure the quality of the process. Typical determinations include conductivity in-line readings in both the feed and permeate streams, which are expressed in terms of "micro MHO's". This data can then be used to calculate the percent salt rejection. A loss in percent salt rejection may or may not be accompanied by changes in other RO performance indicators.
For instance, it is recommended that as a routine monitoring procedure, it is useful to obtain a periodic conductivity profile of each pressure vessel within an RO system. Such monitoring can be used to identify and assess a number of parameters, including fouling of the vessel, operating conditions, mechanical failure, and anticipated clean-up procedures.
The technique most commonly used to sample the permeate, particularly when using spiral wound membrane systems, presently involves having technicians manually probe the vessel with tubing. Commonly a 1/4 inch polypropylene tube is used, which is provided in a sufficient length to traverse the length of the vessel through the permeate line. (See, e.g., Bukay, et at., Ultrapure Water, pp. 62, March/April 1986). Analysis of samples drawn from the tube at various positions within the RO system allows the technicians to identify membrane system irregularities.
In what appears to be the most common practice presently used, the tube is inserted by hand into the full length of the permeate tube. The TDS of the permeate sample from the tubing is measured with a hand-held TDS meter. The tube is withdrawn and the procedure is repeated until a TDS profile of each element is obtained.
Probing is typically accomplished by removing the vessel's product manifolds, on one end of the vessel, or by removing the permeate plug on the opposite end of the vessel. While the RO system is operating at normal pressure water then flows from the permeate tube of the vessel, i.e., in the reverse direction of the normal flow. A tube is inserted and made to traverse the length of the pressure vessel. After a few minutes, to allow the RO system to equilibrate, the TDS of the permeate is measured in an incremental fashion along the length of the vessel.
One significant drawback commonly associated with such probing methods is the fact that there is frequently significant permeate loss and spillage associated with the removal of the permeate plug. Similarly, such sampling processes are particularly inconvenient since they often require two technicians. Technicians typically need to remove the permeate plug from the vessel and keep the permeate plug off for the duration of the sampling process.
The water that is spilled in the course of the sampling process not only adds to the inconvenience, but also affects the operation of the pressure vessel itself. The loss of water leads to a change in the direction of water flow within the vessel, as permeate flows from other units (in a multi-pressure vessel system) or from other portions of the same unit. The present sample process can also lead to inaccuracies, as the result of permeate contaminating the sample region.
It would clearly be desirable to have an apparatus and method for sampling the liquid contents of vessels, and in particular for probing the permeate stream of a pressure vessel, in a manner that avoids these drawbacks, yet that is efficient, effective, and inexpensive.